1. Field of the Invention
The present invention relates to a fingerprint sensor for receiving light reflected from a human finger and outputting an electrical signal regarding fingerprint information and a fingerprint recognition system incorporating the fingerprint sensor, and particularly relates to their optical components.
2. Description of the Prior Art
A prior art fingerprint sensor is disclosed, for example, in a paper entitled “Entry Method of Fingerprint Image with Prism—Comparison between Total Reflection Method and Light-Path Separation Method—”, by Shimizu et al., the Transaction of the Inst. of Electronics and Communication Engineers of Japan, Vol.J68-D, No.3, March 1985, pp. 414-415.
FIG. 27 is a diagram schematically showing a configuration of a prior art fingerprint recognition system utilizing the fingerprint sensor disclosed in the paper. The fingerprint recognition system of FIG. 27 comprises a triangular prism 1 including a first surface 1a on which a human finger 8 is placed, a second surface 1b, and a third surface 1c. The first surface 1a is surrounded by a cover plate 9. The fingerprint recognition system also comprises a light source 3 for irradiating the finger 8 with light, a condenser lens 4 for collecting light reflected by the finger 8, an image-sensing device 5 for receiving the reflected light, a signal processor 6, and a fingerprint recognition section 7. In the fingerprint recognition system, the light source 3 irradiates the finger 8 through the third surface 1c of the triangular prism 1 with light. The light is reflected by the finger 8 placed on the first surface 1a, passes through the second surface 1b, and is collected by the condenser lens 4, thereby directed to the image-sensing device 5.
A disadvantage of the prior art fingerprint recognition system described above is that an image sensed by the image-sensing device 5 is subjected to trapezoidal distortion because the condenser lens 4 and the image-sensing device 5 are disposed at an angle with respect to the finger 8 as a subject to be sensed. FIG. 28A and FIG. 28B are diagrams showing regularly arranged multiple squares as a subject to be sensed and a trapezoidal-distorted image of the regularly arranged multiple squares in the prior art fingerprint recognition system, respectively. In the prior art system, the multiple squares (FIG. 28A) on the first surface 1a is projected as the trapezoidal-distorted image (FIG. 28B) on the image-sensing device 5.
Since the trapezoidal distortion interferes with the fingerprint recognition, a sufficiently long distance must be kept between the finger 8 and the image-sensing device 5 so that the effect of the trapezoidal distortion can be reduced. This is because as the image-sensing device 5 is placed closer to the finger 8, the degree of trapezoidal distortion increases. Therefore, it has been difficult to shrink the size of the fingerprint recognition system or fingerprint sensor.
Another method known to reduce the effect of the trapezoidal distortion is to use a fiber optics plate (FOP) instead of the triangular prism. However, the FOP is a very expensive optical component and has a great volume in comparison with the optical image forming system (the triangular prism and the lens), so that it still has been difficult to downsize the fingerprint recognition system or fingerprint sensor.